Method and apparatus for high solids slurry polymerization

ABSTRACT

An olefin polymerization process wherein monomer, diluent and catalyst are circulated in a continuous loop reactor and product slurry is recovered by means of a continuous product take off. The continuous product allows operating the reaction at significantly higher solids content in the circulating slurry. In a preferred embodiment, the slurry is heated in a flash line heater and passed to a high pressure flash where a majority of the diluent is separated and thereafter condensed by simple heat exchange, without compression, and thereafter recycled. Also an olefin polymerization process operating at higher reactor solids by virtue of more aggressive circulation.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of application Ser. No.10/176,289, filed Jun. 20, 2002, now pending, which is a continuation ofapplication Ser. No. 09/586,370, filed Jun. 2, 2000, which is adivisional of application Ser. No. 08/893,200, filed on Jul. 15, 1997,which issued as U.S. Pat. No. 6,239,235, on May 29, 2001.

BACKGROUND OF THE INVENTION

[0002] This invention relates to the polymerization of olefin monomersin a liquid diluent.

[0003] Addition polymerizations are frequently carried out in a liquidwhich is a solvent for the resulting polymer. When high density (linear)ethylene polymers first became commercially available in the 1950's thiswas the method used. It was soon discovered that a more efficient way toproduce such polymers was to carry out the polymerization under slurryconditions. More specifically, the polymerization technique of choicebecame continuous slurry polymerization in a pipe loop reactor with theproduct being taken off by means of settling legs which operated on abatch principle to recover product. This technique has enjoyedinternational success with billions of pounds of ethylene polymers beingso produced annually. With this success has come the desirability ofbuilding a smaller number of large reactors as opposed to a largernumber of small reactors for a given plant capacity.

[0004] Settling legs, however, do present two problems. First, theyrepresent the imposition of a “batch” technique onto a basic continuousprocess. Each time a settling leg reaches the stage where it “dumps” or“fires” accumulated polymer slurry it causes an interference with theflow of slurry in the loop reactor upstream and the recovery systemdownstream. Also the valve mechanism essential to periodically seal offthe settling legs from the reactor upstream and the recovery systemdownstream requires frequent maintenance due to the difficulty inmaintaining a tight seal with the large diameter valves needed forsealing the legs.

[0005] Secondly, as reactors have gotten larger, logistic problems arepresented by the settling legs. If a pipe diameter is doubled the volumeof the reactor goes up four-fold. However, because of the valvemechanisms involved, the size of the settling legs cannot easily beincreased further. Hence the number of legs required begins to exceedthe physical space available.

[0006] In spite of these limitations, settling legs have continued to beemployed where olefin polymers are formed as a slurry in a liquiddiluent. This is because, unlike bulk slurry polymerizations (i.e. wherethe monomer is the diluent) where solids concentrations of better than60 percent are routinely obtained, olefin polymer slurries in a diluentare generally limited to no more than 37 to 40 weight percent solids.Hence settling legs have been believed to be necessary to give a finalslurry product at the exit to the settling legs of greater than 37-40percent. This is because, as the name implies, settling occurs in thelegs to thus increase the solids concentration of the slurry finallyrecovered as product slurry.

[0007] Another factor affecting maximum practical reactor solids iscirculation velocity, with a higher velocity for a given reactordiameter allowing for higher solids since a limiting factor in theoperation is reactor fouling due to polymer build up in the reactor.

SUMMARY OF THE INVENTION

[0008] It is an object of this invention to produce olefin polymers as aslurry in a liquid diluent utilizing continuous product slurry takeoff;

[0009] It is a further object of this invention to operate a slurryolefin polymerization process in a diluent at a reactor solidsconcentration high enough to make direct continuous product takeoffcommercially viable;

[0010] It is a further object of this invention to operate a slurryolefin polymerization process in a diluent at higher circulationvelocities.

[0011] It is yet a further object of this invention to operate a slurryolefin polymerization process in a diluent in a reaction zone of greaterthan 30,000 gallons; and

[0012] It is still yet a further object of this invention to provide aloop reactor apparatus having a capacity of greater than 30,000 gallonsand having a continuous take off means.

[0013] In accordance with one aspect of this invention, an olefinpolymerization process is carried out at a higher reactor solidsconcentration by means of continuous withdrawal of product slurry.

[0014] In accordance with another aspect of this invention, a loopreactor olefin polymerization process is carried out by operating at ahigher circulation velocity for a given reactor pipe diameter.

[0015] In accordance with another aspect of this invention, a looppolymerization apparatus is provided having an elongated hollowappendage at a downstream end of one of the longitudinal segments of theloop, the hollow appendage being in direct fluid communication with aheated flash line and thus being adapted for continuous removal ofproduct slurry.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In the drawings, forming a part hereof,

[0017]FIG. 1 is a schematic perspective view of a loop reactor andpolymer recovery system;

[0018]FIG. 2 is cross section along line 2--2 of FIG. 1 showing acontinuous take off appendage;

[0019]FIG. 3 is a cross section along line 3--3 of FIG. 2 showing a ramvalve arrangement in the continuous take off assembly;

[0020]FIG. 4 is a cross section of a tangential location for thecontinuous take off assembly;

[0021]FIG. 5 is a side view of an elbow of the loop reactor showing botha settling let and continuous take off assemblies;

[0022]FIG. 6 is a cross section across line 6--6 of FIG. 5 showing theorientation of two of the continuous take off assemblies; FIG. 7 is aside view showing another orientation for the continuous take offassembly;

[0023]FIG. 8 is a cross sectional view of the impeller mechanism;

[0024]FIG. 9 is a schematic view showing another configuration for theloops wherein the upper segments 14 a are 180 degree half circles andwherein the vertical segments are at least twice as long as thehorizontal segments and

[0025]FIG. 10 is a schematic view showing the longer axis disposedhorizontally.

[0026]FIG. 11 is a schematic diagram illustrating a process forseparating polymer from diluent in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

[0027] Surprisingly, it has been found that continuous take off ofproduct slurry in an olefin polymerization reaction carried out in aloop reactor in the presence of an inert diluent allows operation of thereactor at a much higher solids concentration. Commercial production ofpredominantly ethylene polymers in isobutane diluent has generally beenlimited to a maximum solids concentration in the reactor of 37-40 weightpercent. However, the continuous take off has been found to allowsignificant increases in solids concentration. Furthermore, thecontinuous take off itself brings about some additional increase insolids content as compared with the content in the reactor from which ittakes off product because of the placement of the continuous take offappendage which selectively removes a slurry from a stratum where thesolids are more concentrated. Hence concentrations of greater than 40weight percent are possible in accordance with this invention.

[0028] Throughout this application, the weight of catalyst isdisregarded since the productivity, particularly with chromium oxide onsilica, is extremely high.

[0029] Also surprisingly, it has been found that more aggressivecirculation (with its attendant higher solids concentration) can beemployed. Indeed more aggressive circulation in combination with thecontinuous take off, solids concentrations of greater than 50 weightpercent can be removed from the reactor by the continuous take off. Forinstance, the continuous take off can easily allow operating at 5-6percentage points higher; i.e., the reactor can be adjusted to easilyraise solids by 10 percent; and the more aggressive circulation caneasily add another 7-9 percentage points which puts the reactor above 50percent. But, because the continuous take off is positioned to take offslurry from a stratum in the stream which has a higher than averageconcentration of solids, the product actually recovered has about 3percentage points (or greater) higher concentration than the reactorslurry average. Thus the operation can approach an effective slurryconcentration of 55 weight percent or more, i.e. 52 percent average inthe reactor and the removal of a component which is actually 55 percent(i.e. 3 percentage points) higher.

[0030] It must be emphasized that in a commercial operation as little asa one percentage point increase in solids concentration is of majorsignificance. Therefore going from 37-40 average percent solidsconcentration in the reactor to even 41 is important; thus going togreater than 50 is truly remarkable.

[0031] The present invention is applicable to any olefin polymerizationin a loop reactor utilizing a diluent so as to produce a product slurryof polymer and diluent. Suitable olefin monomers are 1-olefins having upto 8 carbon atoms per molecule and no branching nearer the double bondthan the 4-position. The invention is particularly suitable for thehomopolymerization of ethylene and the copolymerization of ethylene anda higher 1-olefin such as butene, 1-pentene, 1-hexene, 1-octene or1-decene. Especially preferred is ethylene and 0.01 to 10, preferably0.01 to 5, most preferably 0.1 to 4 weight percent higher olefin basedon the total weight of ethylene and comonomer. Alternatively sufficientcomonomer can be used to give the above-described amounts of comonomerincorporation in the polymer.

[0032] Suitable diluents (as opposed to solvents or monomers) are wellknown in the art and include hydrocarbons which are inert and liquidunder reaction conditions. Suitable hydrocarbons include isobutane,propane, n-pentane, i-pentane, neopentane and n-hexane, with isobutanebeing especially preferred.

[0033] Suitable catalysts are well known in the art. Particularlysuitable is chromium oxide on a support such as silica as broadlydisclosed, for instance, in Hogan and Banks, U.S. Pat. No. 2,285,721(March 1958), the disclosure of which is hereby incorporated byreference.

[0034] Referring now to the drawings, there is shown in FIG. 1 a loopreactor 10 having vertical segments 12, upper horizontal segments 14 andlower horizontal segments 16. These upper and lower horizontal segmentsdefine upper and lower zones of horizontal flow. The reactor is cooledby means of two pipe heat exchangers formed by pipe 12 and jacket 18.Each segment is connected to the next segment by a smooth bend or elbow20 thus providing a continuous flow path substantially free frominternal obstructions. The polymerization mixture is circulated by meansof impeller 22 (shown in FIG. 8) driven by motor 24. Monomer, comonomer,if any, and make up diluent are introduced via lines 26 and 28respectively which can enter the reactor directly at one or a pluralityof locations or can combine with condensed diluent recycle line 30 asshown. Catalyst is introduced via catalyst introduction means 32 whichprovides a zone (location) for catalyst introduction. The elongatedhollow appendage for continuously taking off an intermediate productslurry is designated broadly by reference character 34. Continuous takeoff mechanism 34 is located in or adjacent to a downstream end of one ofthe lower horizontal reactor loop sections 16 and adjacent or on aconnecting elbow 20.

[0035] The continuous take off appendage is shown at the downstream endof a lower horizontal segment of the loop reactor which is the preferredlocation. The location can be in an area near the last point in the loopwhere flow turns upward before the catalyst introduction point so as toallow fresh catalyst the maximum possible time in the reactor before itfirst passes a take off point. However, the continuous take offappendage can be located on any segment or any elbow.

[0036] Also, the segment of the reactor to which the continuous take offappendage is attached can be of larger diameter to slow down the flowand hence further allow stratification of the flow so that the productcoming off can have an even greater concentration of solids.

[0037] The continuously withdrawn intermediate product slurry is passedvia conduit 36 into a high pressure flash chamber 38. Conduit 36includes a surrounding conduit 40 which is provided with a heated fluidwhich provides indirect heating to the slurry material in flash lineconduit 36. Vaporized diluent exits the flash chamber 38 via conduit 42for further processing which includes condensation by simple heatexchange using recycle condenser 50, and return to the system, withoutthe necessity for compression, via recycle diluent line 30. Recyclecondenser 50 can utilized any suitable heat exchange fluid known in theart under any conditions known in the art. However preferably a fluid ata temperature that can be economically provided is used. A suitabletemperature range for this fluid is 40 degrees F. to 130 degrees F.Polymer particles are withdrawn from high pressure flash chamber 38 vialine 44 for further processing using techniques known in the art.Preferably they are passed to low pressure flash chamber 46 andthereafter recovered as polymer product via line 48. Separated diluentpasses through compressor 47 to line 42. This high pressure flash designis broadly disclosed in Hanson and Sherk, U.S. Pat. No. 4,424,341 (Jan.3, 1984), the disclosure of which is hereby incorporated by reference.Surprisingly, it has been found that the continuous take off not onlyallows for higher solids concentration upstream in the reactor, but alsoallows better operation of the high pressure flash, thus allowing themajority of the withdrawn diluent to be flashed off and recycled with nocompression. Indeed, 70 to 90 percent of the diluent can generally berecovered in this manner. This is because of several factors. First ofall, because the flow is continuous instead of intermittent, the flashline heaters work better. Also, the pressure drop after the proportionalcontrol valve that regulates the rate of continuous flow out of thereactor has a lower pressure which means when it flashes it drops thetemperature lower thus further giving more efficient use of the flashline heaters.

[0038] Referring now to FIG. 2, there is shown elbow 20 with continuoustake off mechanism 34 in greater detail. The continuous take offmechanism comprises a take off cylinder 52, a slurry withdrawal line 54,an emergency shut off valve 55, a proportional motor valve 58 toregulate flow and a flush line 60. The reactor is run “liquid” full.Because of dissolved monomer the liquid has slight compressibility, thusallowing pressure control of the liquid full system with a valve.Diluent input is generally held constant, the proportional motor valve58 being used to control the rate of continuous withdrawal to maintainthe total reactor pressure within designated set points.

[0039] Referring now to FIG. 3, which is taken along section line 3-3 ofFIG. 2, there is shown the smooth curve or elbow 20 having associatedtherewith the continuous take off mechanism 34 in greater detail, theelbow 20 thus being an appendage-carrying elbow. As shown, the mechanismcomprises a take off cylinder 52 attached, in this instance, at a rightangle to a tangent to the outer surface of the elbow. Coming offcylinder 52 is slurry withdrawal line 54. Disposed within the take offcylinder 52 is a ram valve 62 which serves two purposes. First itprovides a simple and reliable clean-out mechanism for the take offcylinder if it should ever become fouled with polymer. Second, it canserve as a simple and reliable shut-off valve for the entire continuoustake off assembly.

[0040]FIG. 4 shows a preferred attachment orientation for the take offcylinder 52 wherein it is affixed tangentially to the curvature of elbow20 and at a point just prior to the slurry flow turning upward. Thisopening is elliptical to the inside surface. Further enlargement couldbe done to improve solids take off.

[0041]FIG. 5 shows four things. First, it shows an angled orientation ofthe take off cylinder 52. The take off cylinder is shown at an angle,alpha, to a plane that is (1) perpendicular to the centerline of thehorizontal segment and (2) located at the downstream end of thehorizontal segment 16. The angle with this plane is taken in thedownstream direction from the plane. The apex for the angle is thecenter point of the elbow radius as shown in FIG. 5. The plane can bedescribed as the horizontal segment cross sectional plane. Here theangle depicted is about 24 degrees. Second, it shows a plurality ofcontinuous take off appendages, 34 and 34 a. Third, it shows oneappendage, 34 oriented on a vertical center line plane of lower segment16, and the other, 34 a, located at an angle to such a plane as will beshown in more detail in FIG. 6. Finally, it shows the combination ofcontinuous take off appendages 34 and a conventional settling leg 64 forbatch removal, if desired.

[0042] As can be seen from the relative sizes, the continuous take offcylinders are much smaller than the conventional settling legs. Yetthree 2-inch ID continuous take off appendages can remove as muchproduct slurry as 14 8-inch ID settling legs. This is significantbecause with current large commercial loop reactors of 15,000-18000gallon capacity, six eight inch settling legs are required. It is notdesirable to increase the size of the settling legs because of thedifficulty of making reliable valves for larger diameters. As notedpreviously, doubling the diameter of the pipe increases the volumefour-fold and there simply in not enough room for four times as manysettling legs to be easily positioned. Hence the invention makesfeasible the operation of larger, more efficient reactors. Reactors of30,000 gallons or greater are made possible by this invention. Generallythe continuous take off cylinders will have a nominal internal diameterwithin the range of 1 inch to less than 8 inches. Preferably they willbe about 2-3 inches internal diameter.

[0043]FIG. 6 is taken along section line 6-6 of FIG. 5 and shows takeoff cylinder 34 a attached at a place that is oriented at an angle,beta, to a vertical plane containing the center line of the reactor.This plane can be referred to as the vertical center plane of thereactor. This angle can be taken from either side of the plane or fromboth sides if it is not zero. The apex of the angle is located at thereactor center line. The angle is contained in a plane perpendicular tothe reactor center line as shown in FIG. 6.

[0044] It is noted that there are three orientation concepts here. Firstis the attachment orientation, i.e. tangential as in FIG. 4 andperpendicular as in FIG. 2 or 7 or any angle between these two limits of0 and 90 degrees. Second is the orientation relative to how far up thecurve of the elbow the attachment is as represented by angle alpha (FIG.5). This can be anything from 0 to 60 degrees but is preferably 0 to 40degrees, more preferably 0 to 20 degrees. Third is the angle, beta, fromthe center plane of the longitudinal segment (FIG. 6). This angle can befrom 0 to 60 degrees, preferably 0 to 45 degrees, more preferably 0-20degrees.

[0045]FIG. 7 shows an embodiment where the continuous take off cylinder52 has an attachment orientation of perpendicular, an alpha orientationof 0 (inherent since it is at the end, but still on, the straightsection), and a beta orientation of 0, i.e. it is right on the verticalcenterline plane of the lower horizontal segment 16.

[0046]FIG. 8 shows in detail the impeller means 22 for continuouslymoving the slurry along its flow path. As can be seen in this embodimentthe impeller is in a slightly enlarged section of pipe which serves asthe propulsion zone for the circulating reactants. Preferably the systemis operated so as to generate a pressure differential of at least 18psig preferably at least 20 psig, more preferably at least 22 psigbetween the upstream and downstream ends of the propulsion zone in anominal two foot diameter reactor with total flow path length of about950 feet using isobutane to make predominantly ethylene polymers. Asmuch as 50 psig or more is possible. This can be done by controlling thespeed of rotation of the impeller, reducing the clearance between theimpeller and the inside wall of the pump housing or by using a moreaggressive impeller design as is known in the art. This higher pressuredifferential can also be produced by the use of at least one additionalpump.

[0047] Generally the system is operated so as to generate a pressuredifferential, expressed as a loss of pressure per unit length ofreactor, of at least 0.07, generally 0.07 to 0.15 foot slurry heightpressure drop per foot of reactor length for a nominal 24 inch diameterreactor. Preferably, this pressure drop per unit length is 0.09 to 0.11for a 24 inch diameter reactor. For larger diameters, a higher slurryvelocity and a higher pressure drop per unit length of reactor isneeded. This assumes the density of the slurry which generally is about0.5-0.6.

[0048] Referring now to FIG. 9 the upper segments are shown as 180degree half circles which is the which is the preferred configuration.The vertical segments are at least twice the length, generally aboutseven to eight times the length of the horizontal segments. Forinstance, the vertical flow path can be 190-225 feet and the horizontalsegments 25-30 feet in flow path length. Any number of loops can beemployed in addition to the four depicted here and the eight depicted inFIG. 1, but generally four or six are used. Reference to nominal twofoot diameter means an internal diameter of about 21.9 inches. Flowlength is generally greater than 500 feet, generally greater than 900feet, with about 940 to 1,350 feet being quite satisfactory.

[0049] Commercial pumps for utilities such as circulating the reactantsin a closed loop reactor are routinely tested by their manufacturers andthe necessary pressures to avoid cavitation are easily and routinelydetermined.

EXAMPLES

[0050] A four vertical leg polymerization reactor using a 26 inchLawrence Pumps Inc. pump impeller D51795/81-281 in a M51879/FAB casingwas used to polymerize ethylene and hexene-1. This pump was comparedwith a 24 inch pump which gave less aggressive circulation (0.66 ft ofpressure drop vs 0.98). This was then compared with the same moreaggressive circulation and a continuous take off assembly of the typeshown by reference character 34 of FIG. 5. The results are shown below.DATA TABLE Description 24 in Pump 26 in Pump 26 in Pump + CTO Date ofOperation Oct 4-9, 1994 May 24-28, 1995 Nov 15-18, 1996 Avg. ReactorSolids 39 45 53 Concentration, wt % Polymer Production 40.1 40.7 39.9Rate, mlbs/hr Reactor Circulation 430 691 753 Pump Power, kw CirculationPump 14.3 22.4 23.7 Pressure Diff, psi Circulation Pump 61.8 92.5 92.4Head, ft Reactor Slurry Flow 39 46 45 Rate, mGPM Reactor Slurry Density,gm/cc 0.534 0.558 0.592 Reactor Temperature, F. 215.6 218.3 217.0Ethylene 4.43 3.67 4.9 Concentration, wt % Hexene-1 0.22 0.17 0.14Concentration, wt % Reactor Heat Transfer 270 262 241 CoefficientReactor Inside 22.0625 22.0625 22.0625 Diameter, inches Reactor Volume,gal 18700 18700 18700 Reactor Length, ft 941 941 941 Pressure Drop perFoot 0.066 0.098 0.098 of Reactor, ft/ft

[0051] As noted above, a flash vessel design which may be used inconjunction with the continuous take off techniques discussed herein isdisclosed in U.S. Pat. No. 4,424,341 to Hanson and Sherk, which isincorporated by reference. For the convenience of the reader, aspects ofthe Hanson and Sherk technique applicable to the present continuous takeoff techniques are reproduced below.

[0052] While the present invention is applicable to any mixture whichcomprises a slurry of polymer solid and diluent, it is particularlyapplicable to the slurries resulting from olefin polymerizations. Theolefin monomers generally employed in such reactions are 1-olefinshaving up to 8 carbon atoms per molecule and no branching nearer thedouble bond than the 4-position. Typical examples include ethylene,propylene, butene-1,1-pentene, and 1,3-butadiene.

[0053] Typical diluents employed in such olefin polymerizations includehydrocarbons having 3 to 12, preferably 3 to 8 carbon atoms permolecule, such as propane, propylene, n-butane, n-pentane, isopentane,n-hexane, toluene, isooctane, isobutane, 1-butene, and the like. In somecases, naphthene hydrocarbons having 5 to 6 carbon atoms in thenaphthenic ring are also used. Examples of such naphthenic hydrocarbonsinclude cyclohexane, cyclopentane, methylcyclopentane, ethylcyclohexane,and the like.

[0054] The temperature to which the slurry is heated for vaporizationwill vary of course depending upon the nature of the diluent, the natureof the polymer, and the temperature of the heat exchange fluid that isused to condense the vaporized diluent. Obviously, the temperature mustbe raised above the dew point of the diluent at the flashing pressure.Further the temperature should be below that of the melting point of thepolymer to preclude accumulation of polymer in the process vessels andto preclude agglomeration of the polymer particles.

[0055] The pressure for the first flash step will likewise varydepending upon the nature of the diluent and the temperature selected.Typically, pressures in the range of about 30 to about 300 psia can beemployed, preferably about 150 to 250 psia.

[0056] The heat exchanging fluid used to condense the vapor from thefirst flash step is, as indicated above, at a temperature in the rangeof about 40° F. to 130° F. A particularly preferred embodiment uses aheat exchange fluid at a temperature of moderate ambient conditions, forexample, temperatures in the range of 60° to 100° F., more preferably86° to 960 F.

[0057] A further understanding of the present invention will be providedby referring to FIG. 11 which illustrates a system comprising anembodiment of the invention.

[0058] In the embodiment illustrated in FIG. 11, the polymerization iscarried out in a loop reactor 110. The polymerization mixture iscirculated by agitator 111. Monomer and diluent are introduced throughconduits 114 and 116, respectively, connected to conduit 113. Catalystis added through conduit 117. Normally catalyst is introduced as asuspension in a hydrocarbon diluent.

[0059] Polymer slurry is removed from the loop to a settling leg 118.The slurry passes from settling leg 118 to conduit 119 and into flashchamber 120. Conduit 119 has an indirect heat exchange means such as aflash line heater 121. The flash chamber 120 as illustrated includes inits lower end a gas distribution plate 122. Heated diluent vaporprovided via conduit 123 is passed into the flash chamber 120 andthrough the distributor plate 122 in such a fashion as to cause afluidized bed of polymer solids to occur in the flash chamber.

[0060] Vaporized diluent exits the flash chamber 120 via conduit 124through which it is passed into a cyclone 125 which separates entrainedpolymer particles from the vapor. Polymer particles separated by thecyclone are passed via line 126 to a lower pressure flash chamber 127.

[0061] The polymer particles in the fluidized bed are withdrawn viaconduit 128 and also passed into the lower pressure flash chamber 127.In flash chamber 127 substantially all the diluent still associated withthe polymer is vaporized and taken overhead via conduit 129 to a secondcyclone 130.

[0062] The major portion of the diluent associated with the polymersolids as they leave settling leg 118 will have been taken to cyclone125 as vapor via conduit 124. The vapor after having a substantial partof any entrained solids removed is passed via line 131 through a filtercapable of removing any remaining polymer fines. The vapor stream isthen split. One portion is passed via conduit 133 through a heatexchanger 134 wherein the vapor is condensed by indirect heat exchangewith a heat exchange fluid. The condensed diluent is then passed to anaccumulator 135 via conduit 136. Any uncondensed vapors and gases can beremoved overhead from the accumulator 135. A pump 137 is provided forconveying the condensed diluent back to the polymerization zone.

[0063] The other portion of the diluent vapor is passed via line 138through a blower 139 which forces the vapor into conduit 123 to provideat least part of the diluent vapor needed to provide the fluidized bedin flash chamber 120. The vapor that is passed into conduit 123 is firstpassed through a heat exchange zone 140 wherein the vapor is heated ifdesired to provide part or all of the heat needed for heating thepolymer slurry provided by conduit 119.

[0064] The polymer solids in the lower pressure flash tank are passedvia line 141 to a conventional conveyor dryer 142 from which the polymercan be packaged or otherwise handled while in contact with theatmosphere.

[0065] The vapors exit the secondary cyclone 130 via line 143 to afilter 144 such as a bag filter capable of removing any substantialamounts of polymer fines. The filter vapor is then passed to acompressor 145 and the compressed vapors are passed through conduit 146to an air-fin cooler 147 wherein a portion of the compressed vapors arecondensed. The remaining vapors are passed through conduit 148 to acondenser 149 where most of the remaining vapors are condensed and thecondensate is passed through conduit 150 to knockout drum 151 or afractionator. The condensed diluent can then be removed via conduit 152and recycled to the polymerization process. Since the major portion ofthe diluent is recovered from the intermediate pressure flash chamber,the load on compressor 145 is much lower than in prior art techniques ofthe type illustrated in U.S. Pat. No. 3,152,872.

[0066] It is important to note that there are many variations of theillustrated embodiment which fall within the scope of the presentinvention. For example, it is within the scope of the present inventionto eliminate the flash line heater 121 and to have all the heat suppliedby the heated diluent vapor that is used to provide a fluidized bed inflash chamber 120. Further, in some instances, it may be desirable tohave the cyclone 125 actually present in the flash chamber rather thanbeing connected to it by a conduit. Still further, it is within thescope of the present invention to eliminate the fluidized bed conceptand to supply all the heat needed by other means such as the flash lineheater 121. In such a modification, there obviously would no longer be aneed for the gas distributor plate 122.

[0067] It is noted that when recycled diluent vapor from the first flashstep is used as the fluidizing medium in the first flash step, it cansometimes lead to alterations in the properties of the polymer since itoften will contain monomer that could react in the flash step. Undersuch circumstances, it is thus preferred to use a substantially pureheated diluent as the fluidizing medium or to eliminate the fluidizedbed concept and use flash line heaters to provide all the necessaryheat.

[0068] In regard to embodiments employing the fluidized bed concept,experiments were conducted to determine the conditions that would bemost suitable for producing a fluidized bed of the polymer particles.The particles employed were polyethylene particles having sphericitiesin the range of about 0.55 to 0.60 as determined by the Ergun equationas disclosed in Zenz, F. and D. Othmer, Fluidization and Fluid-ParticleSystems, New York; Reinhold, 1960, p. 75. The Ergun equation is${\frac{\Delta \quad P}{L}{gc}} = {\frac{150( {1 - {Em}} )^{2}}{{Em}^{3}}\frac{\mu \quad U_{O}}{( {\varphi \quad {dp}} )^{2}}}$

[0069] where:

[0070] ΔP=pressure drop over the bed length.

[0071] L=bed length.

[0072] gc=dimensional constant when units of force such as lbs-force orKg-force are used.

[0073] Em=porosity of packed bed.

[0074] μ=viscosity of flowing gas.

[0075] U_(o)=gas superficial velocity (based on bed cross-sectionalarea).

[0076] φ_(s)=sphericity of the particles.

[0077] dp=mean particle diameter for mixture.

[0078] For the polyethylene fluff particles having sphericities in therange of about 0.55 to 0.60, it was determined that good fluidizationwas obtained with the superficial velocity of the fluidizing gas beingin the range of about 0.4 to 0.8 ft/sec. It was further noted thatslugging of the bed was a problem when the height of the bed was allowedto be more than about 3 times its diameter. Generally, it would bepreferable for the bed height to be no greater than two times itsdiameter.

[0079] The preferred bed diameter and rate of feeding such a polymerslurry can be calculated by the formula:$t = \frac{750\quad \pi \quad D^{3}}{W}$

[0080] where:

[0081] t=Residence time in minutes necessary for desired level ofdiluent separation.

[0082] D=Bed diameter, ft.

[0083] W=Fluff feed rate, lb/hr.

[0084] Rate data obtained during measurement of equilibrium isobutaneabsorption on polymer fluff indicated that 2 to 3 minutes should beadequate for such polyethylene fluff. Thus, for a pilot plant scaleprocess producing 22 pounds per hour of fluff, a bed diameter of atleast about 4 inch would be preferred. For a commercial processproducing 17,500 pounds of fluff per hour, a bed diameter of at leastabout 4 feet would be preferred. Residence times greater than 10 minutesgenerally should not be necessary.

[0085] The following example sets forth typical conditions that can beused in a commercial scale process in employing the present invention.

EXAMPLE

[0086] A typical ethylene homopolymerization process would be thepolymerization conducted at a pressure of about 650 psia and atemperature of about 225° F. The settling leg would be operated toaccumulate and discharge about 55 weight percent solids. An example ofsuch a process would result in a polymer slurry product containing about17,500 pounds per hour of polyethylene and about 14,318 pounds per hourof isobutane diluent. This slurry would then be flashed to 180 psia and180° F. to vaporize the major portion of the diluent. The auxiliary heatnecessary to cause the effluent to be at 180° F. after the pressure dropto 180 psia can be supplied by preheating the effluent, by heatingrecycled fluidizing diluent, or by a combination of the two methods.About 90 percent of the diluent is taken overhead from flash zone 120 at180 psia. Even assuming that there would be a further pressure dropbetween flash zone 120 and accumulator 135, the isobutane diluent couldreadily be condensed against 60° to 80° F. cooling water withoutcompression. The remaining 10 percent of the diluent and the fluff arethen passed into a lower pressure flash tank wherein they are exposed toa pressure in the range of about 20 to 30 psia. The diluent vapor fromthe lower pressure flash tank can then be condensed using compressionand cooling. The use of the preliminary higher pressure tank results ina significantly lower compression load than was required in theconventional process in which slurry was immediately flashed to apressure in the range of 20 to 30 psia.

What is claimed is:
 1. A method of continuously obtaining polymerproduct from an olefin polymerization reactor comprising an endless loopof pipe, the method comprising: circulating within the loop a slurry ofpolymer particles and liquids while maintaining the reactor at a firstpressure above 400 psia; continuously conveying an amount of the polymerparticles from the reactor first through a discharge means located belowa horizontal midline of a cross-section of the reactor pipe and thenthrough a transfer line; and receiving the polymer particles at theinlet of a non-cyclonic primary flash vessel having vertical sidewallsand a conical bottom and maintained at a second pressure less than 25psia, whereupon the particles settle to the bottom of the flash vessel.2. The method of claim 1 in which the reactor is a horizontal loopreactor.
 3. The method of claim 2 in which the discharge means islocated upstream of a reactor-circulating pump in the reactor.
 4. Themethod of claim 1 in which the first pressure is above 600 psia.
 5. Themethod of claim 2 in which the first pressure is from 635 to 675 psiaand the liquids include isobutane and ethylene.
 6. The method of claim 1including controlling the flow through the discharge means in responseto the pressure of the reactor, and adding fresh olefin feedstock to thereactor at a constant rate.
 7. The method of claim 6 in which thereactor is a horizontal loop reactor.
 8. The method of claim 6 in whichthe reactor is a vertical loop reactor.
 9. The method of claim 1 inwhich the particles enter the non-cyclonic primary flash vessel at atangent to the vertical sidewall in the upper half of the vessel. 10.The method of claim 1 further comprising removing a portion of thepolymer particles via an outlet at the bottom of the flash vessel whileretaining an amount of particles sufficient to maintain a dynamic sealbetween the inlet and the outlet of the vessel.
 11. The method of claim1 in which the reactor is a vertical loop reactor.
 12. The method ofclaim 1 1 in which the particles enter the non-cyclonic primary flashvessel at a tangent to the vertical sidewall in the upper half of thevessel.
 13. The method of claim 11 further comprising removing a portionof the polymer particles via an outlet at the bottom of the flash vesselwhile retaining an amount of particles sufficient to maintain a dynamicseal between the inlet and the outlet of the vessel.
 14. The method ofclaim 13 in which polymer particles are removed while maintaining alevel which fills the conical bottom of the vessel.
 15. The method ofclaim 11 including the additional step, before the polymer particlesenter the primary flash vessel, of receiving the polymer particles atthe inlet of an intermediate non-cyclonic flash vessel with an uppersection having vertical sidewalls and the inlet being tangential to thesidewall, a conical bottom with an outlet therein, and operated at athird pressure intermediate between the first and second pressures. 16.The method of claim 15 in which the first pressure is above 600 psia andthe third pressure in the intermediate flash vessel is above 180 psia,and the second pressure in the primary flash vessel is below 25 psia.17. A method of continuously obtaining polymer product from an olefinpolymerization reactor comprising an endless loop of pipe, the methodcomprising: circulating within the loop a slurry of polymer particlesand liquids while maintaining the reactor at a first pressure above 400psia; continuously conveying an amount of the polymer particles from thereactor first through a discharge means located below a horizontalmidline of a cross-section of the reactor pipe and then through atransfer line; receiving the polymer particles at the inlet of anon-cyclonic primary flash vessel having vertical sidewalls and aconical bottom and maintained at a second pressure less than 25 psia,whereupon the particles settle to the bottom of the flash vessel; andincluding the additional step, before the polymer particles enter theprimary flash vessel, of receiving the polymer particles at the inlet ofan intermediate non-cyclonic flash vessel with an upper section havingvertical sidewalls and the inlet being tangential to the sidewall, aconical bottom with an outlet therein, and operated at a third pressureintermediate between the first and second pressures.
 18. The method ofclaim 17 in which the first pressure is above 600 psia and the thirdpressure in the intermediate flash vessel is above 180 psia, and thesecond pressure in the primary flash vessel is below 25 psia.
 19. Amethod of continuously obtaining polymer product from an olefinpolymerization reactor comprising an endless loop of pipe, the methodcomprising: circulating within the loop a slurry of polymer particlesand liquids while maintaining the reactor at a first pressure above 400psia; continuously conveying an amount of the polymer particles from thereactor first through a discharge means located below a horizontalmidline of a cross-section of the reactor pipe and then through atransfer line; receiving the polymer particles at the inlet of anon-cyclonic primary flash vessel having vertical sidewalls and aconical bottom and maintained at a second pressure less than 25 psia,whereupon the particles settle to the bottom of the flash vessel;removing a portion of the polymer particles via an outlet at the bottomof the flash vessel while retaining an amount of particles sufficient tomaintain a dynamic seal between the inlet and the outlet of the vesseland maintaining a level which fills the conical bottom of the vessel.20. A method of continuously obtaining polymer product from an olefinpolymerization reactor comprising an endless loop of pipe, the methodcomprising: circulating within the loop a slurry of polymer particlesand liquids while maintaining the reactor at a first pressure above 400psia; continuously conveying an amount of the polymer particles from thereactor first through a discharge means located below a horizontalmidline of a cross-section of the reactor pipe and then through atransfer line; and receiving the polymer particles at the inlet of anon-cyclonic first flash vessel having vertical sidewalls and a conicalbottom and maintained at a second pressure less than 25 psia, whereuponthe particles settle to the bottom of the first flash vessel.
 21. Themethod of claim 20 in which the reactor is a horizontal loop reactor.22. The method of claim 21 in which the discharge means is locatedupstream of a reactor-circulating pump in the reactor.
 23. The method ofclaim 20 in which the first pressure is above 600 psia.
 24. The methodof claim 21 in which the first pressure is between 635 to 675 psia andthe liquids include isobutane and ethylene.
 25. The method of claim 21in which the first pressure is about 650 psia and the liquids includeisobutane and ethylene.
 26. The method of claim 20 including controllingthe flow through the discharge means in response to the pressure of thereactor, and adding one or more input streams to the reactor at aconstant rate.
 27. The method of claim 26 in which the reactor is ahorizontal loop reactor.
 28. The method of claim 26 in which the reactoris a vertical loop reactor.
 29. The method of claim 20 in which theparticles enter the non-cyclonic first flash vessel at a tangent to thevertical sidewall in the upper half of the vessel.
 30. The method ofclaim 20 further comprising removing a portion of the polymer particlesvia an outlet at the bottom of the first flash vessel while retaining anamount of particles sufficient to maintain a dynamic seal between theinlet and the outlet of the vessel.
 31. The method of claim 20 in whichthe reactor is a vertical loop reactor.
 32. The method of claim 31 inwhich the particles enter the non-cyclonic first flash vessel at atangent to the vertical sidewall in the upper half of the vessel. 33.The method of claim 31 further comprising removing a portion of thepolymer particles via an outlet at the bottom of the first flash vesselwhile retaining an amount of particles sufficient to maintain a dynamicseal between the inlet and the outlet of the vessel.
 34. The method ofclaim 33 in which polymer particles are removed while maintaining alevel which fills the conical bottom of the vessel.
 35. The method ofclaim 31 including the additional step, before the polymer particlesenter the first flash vessel, of receiving the polymer particles at theinlet of a second non-cyclonic flash vessel with an upper section havingvertical sidewalls and the inlet being tangential to the sidewall, aconical bottom with an outlet therein, and operated at a third pressureintermediate between the first and second pressures.
 36. The method ofclaim 35 in which the first pressure is above 600 psia and the thirdpressure in the second flash vessel is above 180 psia, and the secondpressure in the first flash vessel is below 25 psia.
 37. A method ofcontinuously obtaining polymer product from an olefin polymerizationreactor comprising an endless loop of pipe, the method comprising:circulating within the loop a slurry of polymer particles and liquidswhile maintaining the reactor at a first pressure above 400 psia;continuously conveying an amount of the polymer particles from thereactor first through a discharge means located below a horizontalmidline of a cross-section of the reactor pipe and then through atransfer line; receiving the polymer particles at the inlet of anon-cyclonic first flash vessel having vertical sidewalls and a conicalbottom and maintained at a second pressure less than 25 psia, whereuponthe particles settle to the bottom of the first flash vessel; andincluding the additional step, before the polymer particles enter thefirst flash vessel, of receiving the polymer particles at the inlet of asecond non-cyclonic flash vessel with an upper section having verticalsidewalls and the inlet being tangential to the sidewall, a conicalbottom with an outlet therein, and operated at a third pressureintermediate between the first and second pressures.
 38. The method ofclaim 37 in which the first pressure is above 600 psia and the thirdpressure in the second flash vessel is above 180 psia, and the secondpressure in the first flash vessel is below 25 psia.
 39. A method ofcontinuously obtaining polymer product from an olefin polymerizationreactor comprising an endless loop of pipe, the method comprising:circulating within the loop a slurry of polymer particles and liquidswhile maintaining the reactor at a first pressure above 400 psia;continuously conveying an amount of the polymer particles from thereactor first through a discharge means located below a horizontalmidline of a cross-section of the reactor pipe and then through atransfer line; receiving the polymer particles at the inlet of anon-cyclonic first flash vessel having vertical sidewalls and a conicalbottom and maintained at a second pressure less than 25 psia, whereuponthe particles settle to the bottom of the first flash vessel; removing aportion of the polymer particles via an outlet at the bottom of thefirst flash vessel while retaining an amount of particles sufficient tomaintain a dynamic seal between the inlet and the outlet of the vesseland maintaining a level which fills the conical bottom of the vessel.